Cynthiacetus is an extinct genus of basilosaurid early whale that lived during the Late Eocene (Bartonian-Priabonian, 40.4 to 33.9 million years ago.

)[1] Specimens have been found in the southeastern United States and Peru (Otuma Formation).

[2] Cynthiacetus was named after the town of Cynthia, Mississippi, close to where the type specimen for the species C. maxwelli was discovered.

Uhen 2005 erected the genus to avoid the nomen dubium Pontogeneus (which was based on poorly described and now vanished specimens).

[4] The South American species C. peruvianus, the first archaeocete to be described on that continent, mainly differs from C. maxwelli in the number of cuspids in the lower premolars, but it also has the greatest numbers of thoracic vertebrae (20).